Orthogonal multiple access, in which signals do not interfere with each other, is widely used for communication between a base station and user devices (mobile stations, for example) in mobile communication networks. In orthogonal multiple access, different radio resources are allocated to different user devices. Examples of orthogonal multiple access are code division multiple access (CDMA), time division multiple access (TDMA), and orthogonal frequency-division multiple access (OFDMA). For example, in Long Term Evolution (LTE), which has been standardized by the 3GPP, OFDMA is used in downlink communication. In OFDMA, different frequencies are allocated to different user devices.
In recent years, non-orthogonal multiple access (NOMA) has been proposed as a communication scheme between a base station and user devices (see Patent Document 1, for example). In non-orthogonal multiple access, the same radio resource is allocated to different user devices. More specifically, a single frequency is simultaneously allocated to different user devices. When non-orthogonal multiple access is applied to downlink communication, a base station transmits a signal at high transmission power to a user device having a large path-loss, i.e., a user device having a low received signal-to-interference-plus-noise-power ratio (SINR) (generally a user device located at the edge of a cell area). In contrast, a base station transmits a signal at low transmission power to a user device having a small path-loss, i.e., a user device having a high received SINR (generally a user device located at the center of a cell area). As a result, there is interference between a signal received by one user device and a signal directed to another user device.
In such a case, each user device demodulates the signal directed to the user device using power difference. More specifically, the user device first demodulates a signal with the highest reception power. Since the demodulated signal is directed to a user device located nearest to the edge of a cell area, or a user device with the lowest received SINR, the user device located nearest to the edge of a cell area, or the user device with the lowest received SINR ends demodulation since the demodulated signal is directed to the user device located nearest to the edge of a cell area, or the user device with the lowest received SINR. Each of the other user devices removes from the received signal an interference component corresponding to the demodulated signal using an interference canceller, and then demodulates a signal with the second highest reception power. Since the demodulated signal is directed to a user device located second nearest to the edge of the cell area, or a user device with the second lowest received SINR, the user device located second nearest to the edge of the cell area, or the user device with the second lowest received SINR ends demodulation since the demodulated signal is directed to the user device located second nearest to the edge of the cell area, or the user device with the second lowest received SINR. By repeating the demodulation and the removal of a signal with high power in this way, every user device is able to demodulate a signal directed to the user device.
By combining non-orthogonal multiple access with orthogonal multiple access, it is possible to increase the capacity of a mobile communication network, compared with when orthogonal multiple access alone is used. In other words, when orthogonal multiple access alone is used, it is not possible to allocate a particular radio resource (a frequency, for example) simultaneously to user devices. However, when non-orthogonal multiple access is combined with orthogonal multiple access, it is possible to allocate a particular radio resource simultaneously to user devices.